Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, middle distillates, naphtha, and gasoline, among others, by hydroprocessing a hydro-carbonaceous feedstock derived from crude oil or heavy fractions thereof. Hydroprocessing can include, for example, hydrocracking, hydrotreating, hydrodesulphurization and the like. Feedstocks subjected to hydroprocessing may include vacuum gas oils, heavy gas oils, and other hydrocarbon streams recovered from crude oil by distillation. For example, a typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.) and usually at least about 50 percent by weight boiling above 371° C. (700° F.), and a typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydroprocessing concerns reacting the feedstock in the presence of a hydrogen-containing gas with suitable catalyst(s) to convert constituents of the feedstocks to other forms, to extract contaminants from feedstock, etc. In many instances, hydroprocessing is accomplished by contacting the selected feedstock in a reaction vessel or zone with the suitable catalyst under conditions of elevated temperature and pressure in the presence of hydrogen as a separate phase in a substantially three-phase system (i.e., hydrogen gas, a substantially liquid hydrocarbon stream, and a solid catalyst). Such hydroprocessing apparatuses are commonly undertaken in a trickle-bed reactor where the continuous phase throughout the reactor is gaseous.
In such trickle-bed reactors, a substantial excess of the hydrogen gas is present in the reactor to form the continuous gaseous phase. In many instances, a typical trickle-bed hydrocracking reactor requires up to about 1685 Nm3/m3 (10,000 SCF/B of hydrogen at pressures up to 17.3 MPa (2500 psig) to effect the desired reactions. In these apparatuses, because the continuous phase throughout the reactor is the gas-phase, large amounts of excess hydrogen gas are generally required to maintain this continuous phase throughout the reactor vessel. However, supplying such large supplies of gaseous hydrogen at the operating conditions needed for hydroprocessing adds complexity and capital and operating expense to the hydroprocessing apparatus.
Typically, in order to supply and maintain the needed amounts of hydrogen in a continuous gas-phase system, the effluent from a reactor circuit, such as the trickle-bed reactor, is subject to separation into a gaseous component containing hydrogen and a liquid component. A hydrogen recycle gas compressor is used to recirculate the separated hydrogen back to the reactor circuit inlet to assist in supplying the large amounts of hydrogen gas needed to maintain the reactor's continuous gaseous phase. The recycle gas compressor commonly recirculates hydrogen within the hydroprocessing unit in amounts significantly in excess of the hydrogen used by the reactor circuit due to chemical hydrogen consumption. The recycle gas compressor is distinct from a make-up gas compressor which supplies hydrogen to the unit from the general refinery hydrogen supply.
For example, conventional trickle-bed hydroprocessing units typically operate up to about 17.3 MPa (2500 psig) and, therefore, require the use of a high-pressure recycle gas compressor in order to provide the recycled hydrogen at necessary volumes and elevated pressures. Often such hydrogen recycle can be up to about 1685 Nm3/m3 (10,000 SCF/B, and processing such quantities of hydrogen through a high-pressure compressor adds complexity, increased capital costs, and increased operating costs to the hydroprocessing unit. In general, the recycle gas apparatus may represent as much as about 15 to about 30 percent of the cost of a hydroprocessing unit.
In order to eliminate the costly recycle gas compressor, it has been proposed to utilize a two-phase system using a liquid recycle of the processed product stream back through the hydroprocessing units. The recycled product is essentially inert and can act as diluent for the fresh feed and as a hydrogen carrier. Such systems, however, require large volumes of product to provide the desired ratios of the recycled product to the untreated feed. Maintaining such large ratios of recycled product to untreated feed presents difficulty in the design for larger hydroprocessing units. In many instances, the combined recycled product and untreated feed flow could exceed a single train capacity limit of the unit. Thus such units impose additional expense for large capacity recycle pumps and similar apparatuses, as well as related operational issues to permit such large volumetric flows.
While two-phase systems can operate without a costly recycle gas compressor, the reactions in such two-phase systems are generally less efficient, with less contact time between the unconverted oil and the catalyst than similar reactions in the more common substantially three-phase systems. For example, with a given amount of catalyst, the contact time of the unconverted oil in the feed with the catalyst in the substantially three-phase system is significantly greater than the contact time of the unconverted oil with catalyst in the liquid-phase system. Generally due to the diluents in the feed of the liquid-phase systems, the contact time of the unconverted oil with the catalyst is reduced considerably because so much of the feed is diluent. As a result, the reaction rates in the liquid-phase systems are less efficient and reduced from those in a substantially three-phase system with a similar amount of catalyst.